The present disclosure provides novel addition-fragmentation agents for use in low-stress polymerizable compositions. Free-radical polymerization is typically accompanied by a reduction in volume as monomers are converted to polymer. The volumetric shrinkage produces stress in the cured composition, leading to a microcracks and deformation. Stress transferred to an interface between the cured composition and a substrate can cause failure in adhesion and can affect the durability of the cured composition.
The addition-fragmentation agents of this disclosure provide stress relief by including labile crosslinks that can cleave and reform during the polymerization process. Crosslink cleavage may provide a mechanism to allow for network reorganization, relieve polymerization stress, and prevent the development of high stress regions. The instant addition-fragmentation agents may further provide stress relief by delaying the gel point, the point at which the polymerizable composition transitions from a viscous material to an elastic solid. The longer the polymerizable mixture remains viscous, the more time available during which material flow can act to alleviate stress during the polymerization process.
The addition-fragmentation agents provide novel stress-reducing crosslinking agents that have application in dental compositions, thin films, hardcoats, composites, adhesives, and other uses subject to stress reduction. In addition, the addition-fragmentation process results in a chain-transfer event that provides novel polymers that may be further functionalized.